Cisco IOS SPAN and RSPAN

Cisco Catalyst Switches have a feature called SPAN (Switch Port Analyzer) that lets you copy all traffic from a source port or source VLAN to a destination interface. This is very useful for a number of reasons:

• If you want to use wireshark to capture traffic from an interface that is connected to a workstation, server, phone or anything else you want to sniff.
• Redirect all traffic from a VLAN to an IDS / IPS.
• Redirect all VoIP calls from a VLAN so you can record the calls.

The source can be an interface or a VLAN, the destination is an interface. You can choose if you want to forward transmitted, received or both directions to the destination interface.

When you use a destination interface on the same switch as your switch we call it SPAN, when the destination is a remote interface on another switch we call it RSPAN (Remote SPAN).  When using RSPAN you need to use a VLAN for your RSPAN traffic so that traffic can travel from the source switch to the destination switch.

When you use RSPAN you need to use a VLAN that carries the traffic that you are copying. In the picture above you see SW1 which will copy the traffic from the computer onto a “RSPAN VLAN”. SW2 doesn’t do anything with it while SW3 receives the traffic and forwards it to a computer that has wireshark running. Make sure the trunks between the switches allow the RSPAN VLAN.

SPAN and RSPAN are great but there are a couple of things you need to keep in mind…

Restrictions

Both SPAN and RSPAN have some restrictions, I’ll give you an overview of the most important ones:

• The source interface can be anything…switchport, routed port, access port, trunk port, etherchannel, etc.
• When you configure a trunk as the source interface it will copy traffic from all VLANs, however there is an option to filter this.
• You can use multiple source interfaces or multiple VLANs, but you can’t mix interfaces and VLANs.
• It’s very simple to overload an interface. When you select an entire VLAN as the source and use a 100Mbit destination interface…it might be too much.
• When you configure a destination port you will “lose” its configuration. By default, the destination interface will only be used to forward SPAN traffic to. However, it can be configured to permit incoming traffic from a device that is connected to the destination interface.
• Layer 2 frames like CDP, VTP, DTP and spanning-tree BPDUs are not copied by default but you can tell SPAN/RSPAN to copy them anyway.

This should give you an idea of what SPAN / RSPAN are capable of. The configuration is pretty straight-forward so let me give you some examples…

SPAN Configuration

Let’s start with a simple configuration. I will use the example I showed you earlier:

Switch(config)#monitor session 1 source interface fa0/1
Switch(config)#monitor session 1 destination interface fa0/2

You can verify the configuration like this:

Switch#show monitor session 1
Session 1
---------
Type                   : Local Session
Source Ports           :
    Both               : Fa0/1
Destination Ports      : Fa0/2
    Encapsulation      : Native
          Ingress      : Disabled

As you can see, by default it will copy traffic that is transmitted and received (both) to the destination port. If you only want the capture the traffic going in one direction you have to specify it like this:

Switch(config)#monitor session 1 source interface fa0/1 ?
  ,     Specify another range of interfaces
  -     Specify a range of interfaces
  both  Monitor received and transmitted traffic
  rx    Monitor received traffic only
  tx    Monitor transmitted traffic only

Just add rx or tx and you are ready to go. If interface FastEthernet 0/1 were a trunk you could add a filter to select the VLANs you want to forward:

Switch(config)#monitor session 1 filter vlan 1 - 100

This filter above will only forward VLAN 1 – 100 to the destination. If you don’t want to use an interface as the source but a VLAN, you can do it like this:

Switch(config)#monitor session 2 source vlan 1
Switch(config)#monitor session 2 destination interface fa0/3

I am unable to use session 1 for this because I am already using source interfaces for that session. It’s also impossible to use the same destination interface for another session. This is why I created another session number and picked FastEthernet 0/3 as a destination.

hostname Switch
!
monitor session 1 source interface Fa0/1
monitor session 1 destination interface Fa0/2
monitor session 2 source vlan 1
monitor session 2 destination interface Fa0/3
end

So far so good? Let’s look at RSPAN!

RSPAN Configuration

To demonstrate RSPAN I will use a topology with two switches:

The idea is to forward traffic from FastEthernet 0/1 on SW1 to FastEthernet 0/1 on SW2. There are a couple of things we have to configure here:

SW1(config)#vlan 100
SW1(config-vlan)#remote-span

SW2(config)#vlan 100
SW2(config-vlan)#remote-span

First we need to create the VLAN and tell the switches that it’s a RSPAN vlan. This is something that is easily forgotten. Secondly we will configure the link between the two switches as a trunk:

SW1(config)#interface fastEthernet 0/24
SW1(config-if)#switchport trunk encapsulation dot1q
SW1(config-if)#switchport mode trunk

SW2(config)#interface fastEthernet 0/24
SW2(config-if)#switchport trunk encapsulation dot1q
SW2(config-if)#switchport mode trunk

Now we can configure RSPAN:

SW1(config)#monitor session 1 source interface fastEthernet 0/1
SW1(config)#monitor session 1 destination remote vlan 100

This selects FastEthernet 0/1 as the source and VLAN 100 as the destination…

SW2(config)#monitor session 1 source remote vlan 100
SW2(config)#monitor session 1 destination interface fastEthernet 0/1

And on SW2, we select VLAN 100 as the source and FastEthernet 0/1 as its destination. Here’s the output of the show monitor session command:

SW1#show monitor session 1
Session 1
---------
Type                   : Remote Source Session
Source Ports           :
    Both               : Fa0/1
Dest RSPAN VLAN        : 100

SW2#show monitor session 1
Session 1
---------
Type                   : Remote Destination Session
Source RSPAN VLAN      : 100
Destination Ports      : Fa0/1
    Encapsulation      : Native
          Ingress      : Disabled

That’s all there is to it.

hostname SW1
!
vlan 100
 remote-span
!
interface FastEthernet0/24
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
monitor session 1 source interface Fa0/1
monitor session 1 destination remote vlan 100
end

hostname SW2
!
vlan 100
 remote-span
!
interface FastEthernet0/24
 switchport trunk encapsulation dot1q
 switchport mode trunk
!
monitor session 1 source remote vlan 100
monitor session 1 destination interface fastEthernet 0/1
end

I hope these examples have been helpful to you!